Method and apparatus for compression molding plastic articles

ABSTRACT

A method and apparatus for compression molding plastic articles including closures which includes providing coacting sets of tools including a first set for moving a core and core sleeve into engagement with a cavity mold on a second set of tooling. An actuator is provided between the first set of tooling and a fixed upper cam. The second set of tooling includes an associated actuator supporting the cavity mold and associated with a lower fixed cam. A nitrogen cylinder in the second set of tooling provides for control of the compression molding force. In a preferred form, a plurality of sets of tooling are provided in circumferentially spaced relation on a rotating turret supported by a central column. A common manifold supplies the pressure at accurately controlled pressure to each of the nitrogen cylinders. A control system is provided for monitoring and changing the pressure.

This invention relates to a method and apparatus for compression moldingplastic articles including closures.

BACKGROUND AND SUMMARY OF THE INVENTION

It is common to compression mold plastic articles including closures, ascontrasted to injection molding plastic articles. Typical patentscomprise U.S. Pat. Nos. 2,072,536, 2,155,316, 2,218,456, 2,402,462,2,891,281, 3,210,805, 4,296,061, 4,314,799, 4,343,754, 4,355,759,4,497,765, 4,640,673, 4,755,125 and EPO 0 091 653 A2.

In the compression molding of plastic articles, there are inherentvariations that can affect the resultant articles. One such variance isthe manufacturing tolerance applied to the tools. Accordingly, themolding sets on a machine are not identical. Thus, when the tools aremade up in the molding position, the volume of the space between themolding surfaces varies between mold sets. A further variance is theweight and/or volume of the plastic charge that is placed within eachmold set.

Among the objectives of the present invention are to provide a methodand apparatus for compression molding plastic articles includingclosures wherein the forming pressure can be accurately controlled;wherein the forming pressure can be readily adjusted; wherein lateralforces on the tooling are not applied directly to the forming tool;wherein the tooling may be readily replaced; wherein the number and sizeof tool stations can be readily changed; and wherein various kinds andsizes of articles including closures can be readily made by changing thetooling and associated actuating mechanisms; wherein the tooling willcompensate for variations in pellet or charge weight, variations in moldtooling volume in the closed mold position and wherein a substantialoverload such as a double charge of plastic can be readily absorbedwithout overloading the tooling or the overall apparatus.

In accordance with the invention, a method and apparatus for compressionmolding plastic articles including closures includes providing coactingsets of tools including a first set for moving a core and core sleeveinto engagement with a cavity mold relative to a second set of tooling.The first set of tooling includes an actuator between the tooling and afixed upper cam. The second set of tooling includes an actuatorsupporting the cavity mold and associated with a lower fixed cam. A gascylinder charged with atmospheric gas at a predetermined pressure,preferably nitrogen, is provided in the second set of tooling andcontrols the compression molding force. In a preferred form, a pluralityof sets of tooling are provided in circumferentially spaced relation ona rotating turret supported by a central column. A common manifoldsupplies the pressure at accurately controlled pressure to each of thenitrogen cylinders.

In accordance with the invention, each individual gas charged cylinderin association with its individual tooling has a common predeterminedpressure as defined by the system pressure, each cylinder beingconnected in parallel with each of all the tooling cylinders by means ofconnecting pipework and manifolds.

In the preferred form, each of the gas cylinders has a common diameterand consequently will require an equal force to deflect each of thepistons within its respective cylinder against the common systempressure. In the compression molding process, it is preferred that eachmolded part be molded with a constant molding force and that variationsare held to a minimum. This force will be sufficient to fully form thepart but not so high as to cause flashing of the material from theadjoining mold surfaces.

It is well known in the art that volumetric variations occur within thetooling due to manufacturing tolerances and that the plastic materialcharge can vary in weight and volume. Further, it is understood thatoccasional gross errors will occur in the pellet charge volume as aresult of incorrect machine setting, or as a result of double charge ofplastic, or as a result of a prior molded part not being released fromthe mold prior to receiving a further charge of material.

For this reason, it is preferred to include a means to compensate forthe accumulated volumetric variance resulting from the above phenomenawhile not exceeding the preset molding force established by the systempressure previously mentioned.

Further, there is a limit of volumetric variance beyond which theresulting part is nonfunctional as defined by the desired geometry ofthe part being molded.

In a preferred form, each of the first set of tools is advanced a commonfixed stroke toward the respective cavity within the second set of toolsto close the mold and to form the plastic to fill the mold. Normally theplastic charge is within the desired weight tolerance to correctly formthe part and the pressure in the melt builds up during the forming stageuntil the desired molding force is reached and the cavity is deflectedagainst the supporting gas cylinder to limit any further increase inmolding force. By this means, when the desired molding force is reached,the volumetric space within the tooling combination will remain constantfor the duration of the stroke of the first set of tooling.

If the plastic charge was of a size less than the lower tolerance toproduce a functional part and of insufficient size to fill the mold,then the desired molding force would not be achieved prior to thecompletion of the stroke of the first tooling and the resulting partcould have voids, often referred to as "shorts".

Clearly, if a charge of material exceeded the maximum tolerance toproduce a quality part, then it is important to ensure that the toolingvolume is compensated by further deflection of the cavity under thecontrol of its respective gas cylinder in order that the mold force doesnot reach such proportion as to damage the tooling or the machine.

To maintain a constant molding force within each of successive tools, itis preferred that the force compensating means is contained within thetool and thus independent of any external influences and has a minimumof moving parts to reduce friction. It is also preferred not to usemechanical springs for force compensation as they have a variance inspring rating, are subject to failure, and cannot readily be re-set.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an apparatus embodying the invention.

FIG. 2 is a sectional view taken along the line 2--2 in FIG. 1.

FIG. 3 is a sectional view similar to FIG. 2 with parts being removed.

FIG. 4 is a fragmentary sectional view taken along the line 4--4 in FIG.1.

FIG. 5 is a partly diagrammatic horizontal sectional view.

FIG. 6 is a vertical sectional view of one set of upper and lowertooling.

FIG. 7 is a fragmentary sectional view on an enlarged scale of the lowerassembly of the set of tooling shown in FIG. 6.

FIGS. 8-11 are fragmentary sectional views of one set of tooling duringvarious stages of compression molding.

FIG. 12 is a fragmentary side elevational view of a portion of thetooling shown in FIG. 7.

FIG. 13 is a fragmentary sectional view taken along the line 13--13 inFIG. 12.

FIG. 14 is a schematic diagram of the nitrogen system utilized in theinvention.

FIG. 15 is a schematic diagram of a modified system.

DESCRIPTION

Referring to FIGS. 1-3, the method and apparatus embodying the inventionis shown as applied to a rotary apparatus that includes a base 20,supporting a column 21 on which a turret or carousel 22 is rotatablymounted by upper and lower tapered bearings 23, 24. Turret 22 includesvertically spaced supports comprising an annular upper support 22a, anannular support 22b, and a lower annular support 22c. A plurality ofupper support segments 25 are mounted on upper support 22a and abut todefine a ring. A plurality of lower segments 26 are mounted on theintermediate and lower supports 22b, 22c and abut to define a ring. Eachsegment 25 supports one or more of circumferentially spaced sets ofactuators 34. Each segment 26 supports one or more actuators 52 adjacentthe lower end of the apparatus. The actuators 34 are mounted forvertical movement in housings 34a mounted upon support segments 25which, in turn, are supported on an upper ring mount 22a (FIGS. 2, 3).

An upper set of tooling 27 is associated with each actuator 34 andincludes a movable assembly 27a mounted on the lower end of actuator 34and a fixed assembly 27b mounted on a portion 26a of segment 26 that isfixed to support 22b (FIG. 6). A fixed annular cam 29 is supported bycolumns 30 (FIGS. 1, 4) and is associated with the upper actuators 34.Actuator 34, in turn, has a cam roller 35 at its upper end for engagingthe cam 29.

A set of lower tooling 28 is mounted on each lower actuator 52. A fixedannular cam 31 is supported on base 20 and is associated with the loweractuators 52.

Each lower assembly 28 of tooling includes a female mold assembly 51 anda cooling water manifold 51a. Each actuator 52 has a roller 60 on itslower end for engaging cam 31 (FIG. 6).

Referring to FIG. 6, the fixed assembly 27b is mounted on a segmentportion 26a of the segment 26. The movable assembly 27a comprises a moldplunger or core 41, a spring loaded core sleeve 42 urged upwardly bysprings 45 and a stripper sleeve 44 urged downwardly by stripper springs43. The core 41 is made in several sections and defines a male mold.

Referring to FIGS. 6 and 7, each actuator 52 is mounted in a supportbody 50 forming a part of segment portion 26b. The mold 51 has limitedrelative movement with respect to the actuator 52 and is yieldinglyurged upwardly by cavity springs 53 which act upon lift pins 54. Thelower actuator 52 further includes a plunger 55 that engages a holdingspring plug 56 which in turn engages the piston 57 of a nitrogencylinder 39. A centering spring 59 is interposed between the holdingspring plug 56 and the plunger 55. The nitrogen cylinder 39 is providedwith nitrogen at an accurately determined pressure supplied to the areaor chamber 61 below the nitrogen chamber 39 through a connector 62having an orifice.

In this tooling style, the molding force is applied through the coresleeve 42 and the core 41 attached to the core sleeve 42 by a lostmotion connection, the core 41 being biased upward by the array ofsprings 45. The upper cam 29 is fixed and the core sleeve 42 thus movesdownward a fixed stroke as controlled by the upper cam 29.

Referring to FIGS. 12 and 13, provision is made for rapid removal of thecavity mold 51 and comprises a yoke 80 having spaced arms 81 that engageannular groove 83 in the actuator 52 and groove 82 in the cavity mold51. Grooves 83 in actuator 52 are snugly engaged by arms 81. Groove 82in cavity mold 51 has a greater axial width such that the cavity mold 51has limited axial movement relative to the actuator 52. Spring loadedplungers 84 on the arms 81 extend inwardly to engage the periphery ofmold 28. Yoke 80 includes a handle 85 to facilitate grasping the yoke80.

The one piece cavity mold 51 is located on the lower tooling actuator52, but is free to move axially a limited amount relative to actuator 52by the construction shown in FIGS. 12 and 13 and the cavity mold 51 isheld up by the array of springs 53 within the lower tooling actuator 52.These springs 53 are limited in stroke by the lift pins 54, which bottomout on a lift pin retainer 54a. The cavity mold 51 rests on the plunger55, which is retained to limit its upward travel. The plunger 55contacts the holding spring plug 56 and a spring 59 is fitted betweenthe two components. The holding spring plug 56, in turn, contacts thepiston rod 57 of nitrogen cylinder 39.

Referring to FIG. 7, nitrogen cylinder 39 is normally fully outstroked,maintaining the holding spring plug 56 against the locating stop surface52a in the lower tooling actuator 52. The plunger 55, when free, will bemoved up by the center spring 59 until it reaches a stop surface 52bwithin the lower tooling actuator 52.

Referring to FIG. 6, provision is made for lowering the upper actuator34 and comprises the first roller 35 on the upper end thereof forengaging the upper cam 29 to cause the downward movement of the malemold assembly 32. In addition, a second roller 70 is provided forrotation about the same axis as roller 35 that engages a second fixedupper cam 72 (FIG. 2) for lifting the upper tooling 27 during theoperating cycle in order.

Referring to FIG. 6, provision is made for lifting the lower actuator 52and comprises a roller 60 on actuator 52 that engages the lower cam 31.In addition, a second roller 74 is provided for rotation about the sameaxis as roller 60 and engages a second fixed lower cam 77 for insuringthat the cavity mold 51 is in its lowest position to receive a pellet ofplastic.

OPERATION

Referring to FIGS. 6-11, the operation includes a series of steps:

Position #1 Open

FIGS. 6-7 show the tooling open condition with the upper tool assembly27 up and the lower tool assembly 28 down.

Position #2 Closing of Mold

As shown in FIG. 8, after a charge or pellet of thermoplastic materialis delivered to the female mold, the lower tooling actuator 52 is raiseda fixed stroke by the lower cam 31, and the cavity 51 contacts thestripper sleeve 44 (FIG. 9). The stripper sleeve springs 43 initiallyoverpower the weaker cavity springs 53, and subsequently the smallspring 59 (FIG. 6) in the plunger 55 until the clearances are taken upand the nitrogen cylinder 39 opposes any further compression of thecavity. At this point the stripper sleeve 44 is forced upward againstsprings 43 for the remainder of the lower tooling upward stroke.

Position #3 Molding

Referring to FIG. 10, the upper tooling is now lowered a fixed stroke toform the part, as controlled by the fixed upper forming cam 29. When theforming pressure increases to equal the force of the nitrogen cylinder39, then the piston 57 of the nitrogen cylinder 39 moves to limit themolding force on the tooling. This forces the cavity 51 to lower, andcauses the stripper sleeve springs 43 to move the stripper sleeve 44downward in unison with the cavity 51, until the upper tool stroke iscompleted.

Position #3 Holding

Referring to FIG. 11, a holding pressure is achieved through theparallel sections of the cams 29, 31, under control of the nitrogencylinder 39. No relative axial movement of the tooling takes placeduring this phase.

Position #4 Lowered Holding Force Section

Referring to FIG. 11, a lesser holding force is achieved by a reliefstep in the upper cam 29 only, which allows the core sleeve 41 to becomefree from the cam 29. This causes the piston 57 of nitrogen cylinder 39to outstroke until the holding spring plug 56 bottoms out. The cavity 51is simultaneously lifted by the nitrogen cylinder 39, and the coresleeve 42 along with the core 41 are lifted by the cavity 51. Theholding force is now maintained by the weight of the upper actuator 34and the tooling pin assembly. The cavity springs 53 in the loweractuator 52 play no role in this part of the cycle and remain compressedby the higher rated stripper sleeve springs 43. The small spring 59 onthe inner shaft 55 is also ineffective at this time.

Position #5 Cavity Stripping

During stripping of the closure from cavity 51, the lower toolingactuator 52 is lowered, leaving the closure on the molding core 41. If athreaded closure has been molded, then normally the stripper sleevesprings 43 would not overcome the closure strip force required, andwould remain compressed, and the stripper sleeve 44 would not move down.

The core sleeve 42 is subsequently withdrawn by direct upward cam actiondue to engagement of roller 70 with second upper cam 71 (FIG. 2). Theresistance of the closure to stripping from the threads causes the core41 to stay down, until its lost motion with the core sleeve 42 is takenup. At this time, the core 41 is also pulled up by the action of thecore sleeve 42, causing the core sleeve 42 to compress the array of coresleeve springs 45, and the part to be stripped from the core 41 as thestripper sleeve springs 43 overcome the stripping force.

Referring to FIG. 14, the control system for supplying nitrogen to eachof the cylinders can be more readily understood by reference to thisschematic. The designation LOAD represents cylinders which are connectedto a rotary coupling R (FIG. 2) on the turret 22 that, in turn, isconnected through a pressure control module and a pressure regulator andshut-off valve to a source of nitrogen such as a gas cylinder. Lines Lsuch as hoses extend from the rotary coupling R (FIG. 1) to couplings Cof the nitrogen cylinders 39 (FIG. 6). In the preferred form, thepressure control module includes a solenoid operated valve to exhaustgas to atmosphere, and a second solenoid valve to admit pressurized gasfrom the nitrogen gas cylinder. The interface is a computer andinteractive screen to allow an operator to select a desired systempressure setting by direct input, or the setting could be establishedautomatically by a menu selection from the computer. Preferably, thepressure signal reference to the logic control represents a high and alow pressure limit, and the logic control continually compares thefeedback signal from a pressure transducer representing the actualsystem pressure at any one time. When a pressure is detected above thehigh limit setting, the logic control energizes the first solenoid valveto exhaust gas from the system until the system pressure is withinlimits. Conversely, a system pressure lower than the low limit causesthe second solenoid valve to operate and to admit high pressure nitrogenfrom the gas cylinder until the system pressure is again within thepreset limits.

Although the above description represents a preferred form of thepressure control module, other control systems may be used in which thiscontrol is achieved, and it it not intended to limit the scope of theinvention.

FIG. 5 is a composite sectional view at various horizontal sections. Inthe lower left quadrant of FIG. 5, there is shown how a plurality ofsegments 25 are mounted on the turret and each supports the actuators 34of five sets of upper tooling. In the lower left quadrant, there isshown how a plurality of segments 25 each supporting five sets of uppertooling 27 on the actuators. In the upper left quadrant there is shownhow a plurality, shown as five, of the actuators 52 of lower sets oflower tooling 28 are provided on segments 33. Thus, if it is desired tovary the size or number of sets of tooling, the segments can supportmore or less sets depending on the size.

During normal operation, the molding force control is achieved withminimal compression of the gas cylinder, for example, on the order of0.030 in. This control is maintained despite small variations in thevolumetric capacity of the closed molds, and despite small variations inthe weight of the delivered pellets. On the occasion of inadvertentloading of double pellets into a particular mold tool or due toincorrect setting of the mean pellet weight delivery for the particularmold, then the gas cylinder within the affected tooling willindividually compress to absorb the additional stroke of the cavity andcould compress to limit the molding force to an amount as preset by thegas system pressure up to an amount, for example, of approximately 0.500in. If the pellet weight is outside the specified range for the partbeing molded, then the resulting part will be defective, but the machineloading will be limited by the molding force control described above,and thus machine damage will be eliminated.

Further, inasmuch as the cylinders within the tools are linked by acommon system, and inasmuch as the system volume is much greater thanthe change in volume of the system during normal forming operation, (inexcess of 1000:1) then the system pressure is substantially constant andeach tooling is consequently controlled to the same preset moldingforce.

Thus, each set of tooling is controlled fully throughout the moldingcycle since the same predetermined force is applied to a set of toolingat all times. In addition, each set of tooling is controlled withoutadversely affecting adjacent or other sets of tooling.

The advantages of the present invention can thus be summarized asfollows:

a) Precise force control is provided on each set of tooling.

b) The predetermined force is applied to each set of tooling during theforming and holding portions of each cycle.

c) The force is substantially constant at all times.

d) The pressure control of one set of tooling does not adversely affectother sets of tooling.

e) The use of separate actuators which eliminates side load on the uppertooling.

f) The ability to readily change molding tooling.

g) The ability to readily change the number and size of tooling to formarticles of various sizes and weights.

h) The use of continuous cams which require minimal maintenance.

Although in a preferred form gas cylinders are provided, in accordancewith another aspect of the invention the cylinders may comprisehydraulic fluid cylinders supplied through a rotary coupling as shownschematically in FIG. 15. The hydraulic cylinders designated as LOAD areconnected through the rotary coupling to a gas pressurized accumulatorand the system is supplied with oil from a low volume pump with anadjustable pressure limiting valve. The gas charged accumulator providesfor rapid changes in system volume as a result of piston displacementwithin the respective cylinders and avoids the need for a larger volumepump. If the gas charged accumulating is of sufficient capacity, thenthe resulting pressure control within the system would be substantiallyconstant.

This system could function in a similar manner to the preferred form ofthe invention and the pressure could be remotely controlled by using aservo pilot operated pressure relieving valve. However, it has adisadvantage in situations where possible oil contamination is aconcern.

Hydraulic systems without the gas charged accumulator would not be aseffective as the preferred form of the invention as the hydraulic fluidis substantially incompressible and cannot be controlled or function aseffectively as a gas system wherein the fluid is compressible.

It can thus be seen that there has been provided a method and apparatusfor compression molding plastic articles including closures wherein theforming pressure can be accurately controlled; wherein the formingpressure can be readily adjusted; wherein lateral forces on the toolingare not applied directly to the forming tool; wherein the tooling may bereadily replaced; wherein the number and size of tool stations can bereadily changed; and wherein various kinds and sizes of articlesincluding closures can be readily made by changing the tooling andassociated actuating mechanisms; wherein the tooling will compensate forvariations in pellet or charge weight, variations in mold tooling volumein the closed mold position and wherein a substantial overload such as adouble charge of plastic can be readily absorbed without overloading thetooling or the overall apparatus.

We claim:
 1. A method for compression molding a plastic articlecomprisingproviding a first tool assembly having a male mold associatedtherewith, providing a second tool assembly having a cavity moldassociated therewith, providing a first fixed cam for moving the firsttool assembly relative to said second assembly and a second fixed camfor moving said second tool assembly relative to said first assembly,interposing a fluid cylinder comprising a fluid filled chamber and apiston on one of said tool assemblies interposed between one of saidfixed cams and the associated tool assembly for urging said one toolassembly toward said other tool assembly to provide a constant limitingmolding force during the full movement of the associated tooling underthe actuation of said cams, providing said fluid in said fluid cylinderat a predetermined pressure to provide a constant limiting molding forceduring the forming of the plastic article of the associated toolingunder the actuation of said cams, providing a charge of extrudate to thecavity of the cavity mold, moving the first and second assemblies underthe action of said fixed cams to move the first assembly and second moldassembly toward one another to close the mold and provide a constantlimiting molding force on the charge to compress the charge to form anarticle, providing an array of sets of said first tool assemblies, saidassociated second tool assemblies and said associated fluid cylindershaving the pressure therein maintained at said predetermined pressure toprovide a constant limiting molding force for each set of toolingassemblies unaffected by the other sets of tool assemblies during thefull movement of the associated set under the actuation of said cams,moving said sets of tool assemblies successively in an endless path pasta station wherein a charge of extrudate is delivered successively to acavity mold, and thereafter moving said sets of tooling successivelypast the fixed cams such that the cams cause each set for first toolassembly and second tool assembly to close the mold and provide aconstant molding force on the charge to compress the charge to form anarticle, and connecting the fluid cylinder to a common source of fluidunder said predetermined pressure.
 2. The method set forth in claim 1including interposing said fluid cylinder between said second toolassembly and said second fixed cam.
 3. The method set forth in claim 2wherein said step of interposing said fluid cylinder comprises providinga gas cylinder.
 4. The method set forth in claim 2 wherein said step ofinterposing said fluid cylinder comprises providing a nitrogen cylinder.5. The method set forth in claim 2 wherein said step of interposing saidfluid cylinder comprises providing a hydraulic cylinder, said step ofproviding said fluid at a predetermined pressure in said fluid cylindercomprises providing accumulator means connected with said hydrauliccylinder.
 6. The method set forth in claim 3 including sensing thepressure in said gas cylinders and producing a signal and controllingsaid pressure in response to variation of the pressure signal from apredetermined pressure.
 7. The method set forth in claim 6 includingproviding a rotary coupling between said source and said gas cylinders.8. The method set forth in claim 7 whereby said step for sensing saidpressure is performed by sensing the pressure between said source andsaid rotary coupling.
 9. The method set forth in claim 1 wherein thestep of providing said first tool assembly comprises providing a plungerand the step of providing said second tool assembly comprises providinga cavity for forming a closure having a base wall and a peripheralskirt.
 10. The method set forth in claim 1 including providing a secondfixed upper cam for lifting the first tool assembly.
 11. The method setforth in claim 10 including providing a second fixed lower cam forlowering said second tool assembly.
 12. The method set forth in claim 11including providing an upper actuator interposed between said secondfixed upper cam and said first tool assembly and movable by said secondfixed upper cam toward said second tool assembly.
 13. The method setforth in claim 12 wherein said step of providing said first toolassembly includes providing a movable assembly mounted on said upperactuator and a fixed assembly.
 14. The method set forth in claim 13including providing a lower actuator interposed between said secondfixed lower cam and said second tool assembly and movable by said secondfixed lower cam toward said first tool assembly.
 15. The method setforth in claim 14 including mounting said cavity mold for limited axialmovement with respect to the lower actuator.
 16. The method set forth inclaim 15 including yieldingly urging said cavity mold upwardly.
 17. Themethod set forth in claim 16 including providing said lower actuatorwith a plunger yieldingly urged toward said cavity mold.
 18. The methodset forth in claim 17 including providing a plug interposed between saidpiston of said fluid cylinder and said plunger.
 19. The method set forthin claim 18 including providing centering spring means interposedbetween said plug and said plunger.
 20. The method set forth in claim 19including providing a yoke engaging said cavity mold, said yoke havingspaced arms, providing said cavity mold with a groove, providing saidactuator with an annular groove having a greater axial width than saidarms,causing said arms to engage said groove on said lower actuator andsaid groove on said cavity mold such that the cavity mold has limitedaxial movement relative to said actuator.
 21. The method set forth inclaim 20 including providing said arms with means thereon yieldinglyengaging the periphery of said cavity mold.
 22. The method set forth inclaim 21 including providing handle means on said yoke.
 23. The methodset forth in claim 14 including providing means removably mounting saidcavity mold on said lower actuator.
 24. The method set forth in claim 3including providing a control system for supplying gas to said gascylinders including providing a source of gas under pressure,providingcontrol means for sensing the pressure in each said cylinder, comparingthe pressure with a predetermined maximum pressure value and apredetermined minimum pressure value and controlling the pressure whenthe pressure exceeds the predetermined maximum pressure or is below saidpredetermined minimum pressure.
 25. The method set forth in claim 24including providing said control means with a pressure control modulefor sensing said pressures and logic control means providing electricalsignals to said control module corresponding to said maximum and minimumpressures.
 26. The method set forth in claim 24 including providing arotary coupling connecting to said control means and each said gascylinder.
 27. The method set forth in claim 5 including providing arotary coupling interposed between said accumulator means and each ofsaid hydraulic cylinders.
 28. An apparatus for compression molding aplastic article comprisinga first mold assembly having a male moldassociated therewith, a second mold assembly having a cavity moldassociated therewith, a first fixed cam for moving said first moldassembly relative to said second mold assembly, a second fixed cam formoving said second mold assembly relative to said first mold assembly, afluid cylinder having a fluid filled chamber at a predetermined pressureand a piston on one of said tool assemblies interposed between one ofsaid first fixed cam and said first mold assembly and said second fixedcam and said second mold assembly and urging said one tool assemblytoward said other tool assembly interposed between to provide a constantlimiting molding force during the full movement of the associatedtooling under the actuation of said cams, an array of sets of first toolassemblies, associated second tool assemblies, and associated fluidcylinders having the pressure therein maintained at said predeterminedpressure to provide a constant limiting molding force for each set oftooling assemblies unaffected by the other sets of tooling assemblies,means for moving said sets of tooling assemblies successively in anendless path past a station wherein a charge of extrudate is deliveredsuccessively to a cavity mold, and thereafter successively past saidfixed cams to cause each set of first tool assembly to close the moldand provide a constant limiting molding force on the charge to compressthe charge to form into an article, and means connecting the fluidchambers of said fluid cylinders to a source of fluid pressure.
 29. Theapparatus set forth in claim 28 wherein said fluid cylinder isinterposed between the second tool assembly and said second fixed cam.30. The apparatus set forth in claim 29 wherein said fluid cylindercomprises a gas cylinder.
 31. The apparatus set forth in claim 29wherein said fluid cylinder comprises a nitrogen cylinder.
 32. Theapparatus set forth in claim 29 wherein said fluid cylinder comprises ahydraulic cylinder.
 33. The apparatus set forth in claim 29 wherein saidfirst tool assembly comprises a plunger and said second tool assemblycomprises a cavity for forming a closure having a base wall andperipheral skirt.
 34. The apparatus set forth in claim 30 includingmeans sensing the pressure in said gas cylinders and producing a signaland means for controlling the pressure in response to variation of thepressure signal from a predetermined pressure.
 35. The apparatus setforth in claim 34 including a rotary coupling between said source andsaid gas cylinders.
 36. The apparatus set forth in claim 35 wherein saidmeans for sensing pressure is positioned to sense the pressure between asource and said rotary coupling.
 37. The apparatus set forth in claim 30including a second fixed upper cam for lifting the first tool assembly.38. The apparatus set forth in claim 37 including a second fixed lowercam for lowering said second tool assembly.
 39. The apparatus set forthin claim 38 including an upper actuator interposed between said secondfixed upper cam and said first tool assembly and movable by said secondfixed upper cam toward said second tool assembly.
 40. The apparatus setforth in claim 39 wherein said first tool assembly comprises a movableassembly mounted on said upper actuator and a fixed assembly.
 41. Theapparatus set forth in claim 40 including a lower actuator interposedbetween said second fixed lower cam and said second tool assembly andmovable by said second fixed lower cam toward said first tool assembly.42. The apparatus set forth in claim 41 including means for mountingsaid cavity mold for limited axial movement with respect to the loweractuator.
 43. The apparatus set forth in claim 42 including meansyieldingly urging said cavity mold upwardly.
 44. The apparatus set forthin claim 43 wherein said lower actuator includes a plunger yieldinglyurged toward said cavity mold.
 45. The apparatus set forth in claim 43including a plug interposed between said piston of said fluid cylinderand said plunger.
 46. The apparatus set forth in claim 43 includingcentering spring means interposed between said plug and said plunger.47. The apparatus set forth in claim 41 including a yoke engaging saidcavity mold, said yoke having spaced arms, said cavity mold having agroove, said actuator having an annular groove having a greater axialwidth than said arms, said arms engaging said groove on said loweractuator and said groove on said cavity mold such that the cavity moldhas limited axial movement relative to said actuator.
 48. The apparatusset forth in claim 47 wherein said arms have means thereon yieldinglyengaging the periphery of said cavity mold.
 49. The apparatus set forthin claim 48 including handle means on said yoke.
 50. The apparatus setforth in claim 30 including means removably mounting said cavity mold onsaid lower actuator.
 51. The apparatus set forth in claim 28 whereinsaid cylinders are gas cylinders, a control system for supplying gas tosaid gas cylinders comprising including a source of gas under pressure,control means for sensing the pressure in each said cylinder, comparingthe pressure with a predetermined maximum pressure value and apredetermined minimum pressure value and controlling the pressure whenthe pressure exceeds the predetermined maximum pressure or is below saidpredetermined minimum pressure.
 52. The apparatus set forth in claim 51wherein said control means includes a pressure control module forsensing said pressures and logic control means providing electricalsignals to said control module corresponding to said maximum and minimumpressures.
 53. The apparatus set forth in claim 51 wherein said controlsystem includes a rotary coupling connected to said control means andeach said gas cylinder.
 54. The apparatus set forth in claim 28 whereinsaid cylinders are hydraulic cylinders including accumulator meansconnected to a source of hydraulic fluid and said hydraulic cylinders.55. The apparatus set forth in claim 54 including a rotary couplinginterposed between said accumulator means and each of said hydrauliccylinders.